1. Field of Use
This invention relates generally to a vacuum lifting device suspendable from a crane and operable to lift an object such as a large sheet of material.
2. Description of the Prior Art
Manually handling and moving large sheets of material in warehouses and factories can be difficult, labor-intensive and sometimes dangerous. For example, some large plywood sheets are heavy. Comparably sized sheets of wood and plastic paneling, while lighter in weight, are quite flexible. Large sheets of metal from which parts are punched and stamped and made of iron, steel, brass and copper or the like, are heavy and extremely flexible. Sheets of stone, such as decorative marble or the like, are rigid but extremely heavy. Therefore, overhead cranes are commonly employed to handle and move sheet material and various types of load-handling devices are suspended from the crane loadline to grip the sheets. Such load gripping devices include, for example, various types of slings, electromagnets and suction cup arrangements, but each type has its drawbacks, especially in cases where only a single sheet at a time is to be lifted and moved. For example, slings still require manual attachment and detachment to the sheet. Electromagnets are only capable of directly lifting a sheet made of magnetizable metal, such as iron or steel, and, unless the magnetic field is precisely controlled, lift not only the uppermost sheet from a stack but also one or more sheets immediately below the uppermost sheets. Suction cup gripping devices are relatively limited as regards the weight of the sheet that can be safely lifted and, furthermore, mechanical force must be applied to break the grip.